Cellular Respiration
I. Chemical Pathways
A. Food provides living things with the chemical building blocks they
need.
B. Food serves as a source of raw materials from molecule synthesis
and most importantly serves as a source of energy.
II. Chemical Energy and Food
A. How much energy is actually present in food?
1. One gram of sugar glucose (C6H12O6) when burned in the presence
of oxygen, releases 3811 calories of heat energy.
2. A calorie is the amount of energy needed to raise the temperature
of 1 gram of water 1oC.
3. The Calorie (capital “C”) that is used on food labels is a
kilocalorie.
B. The first step in cellular respiration is glycolysis.
1. Glycolysis releases only a small amount of energy.
2. If O2 is present, glycolysis leads to other pathways that release a
great deal of energy.
III. Cellular Respiration: An Overview
A. In the presence of oxygen, glycolysis is followed by the Krebs
cycle and the electron transport chain.
B. Cellular respiration is made up of 3 steps:
1. Glycolysis
2. Krebs cycle
3. Electron transport chain
C. Cellular respiration is the process that releases energy by breaking
down glucose and other food molecules in the presence of oxygen.
1. 6O2 + C6H12O6
6CO2 + 6H2O + Energy
2. Oxygen + glucose (yields) Carbon Dioxide + Water + Energy
D. Cellular respiration is complex.
E. Living cells have to control the energy released.
F. The cell needs to find a way to trap those little bits of energy by
using them to make ATP.
G. Each of the 3 stages of cellular respiration capture some of the
chemical energy available in food molecules and uses it to produce
ATP.
IV. Glycolysis
A. Glycolysis is the process in which one molecule of glucose is
broken in half, producing two molecules of pyruvic acid, which is a 3carbon compound.
B. ATP Production
1. Even through glycolysis is an energy-releasing process, the cell
needs a little energy to get things going.
2. In the beginning, 2 molecules of ATP are used up.
3. Although the cell puts 2 ATP molecules into its “account” it ends
up with 4 ATP.
4. This gives you a net gain of 2 ATP molecules.
C. Although the energy yield from glycolysis is small, the process is
so fast that cells can produces thousands of ATP molecules in just a
few milliseconds.
D. Glycolysis does not require oxygen.
V. Fermentation
A. When oxygen is not present, glycolysis is followed by a different
pathway.
B. Fermentation releases energy from food molecules by producing
ATP in the absence of oxygen.
C. Because fermentation does not require oxygen, it is said to be an
anaerobic process.
1. The term anaerobic means “not in air”
D. The two main types of fermentation are:
1. Alcoholic fermentation
Yeasts and a few other microorganisms use alcoholic
fermentation, forming ethyl alcohol and carbon dioxide as wastes.
a)
b)
Pyruvic acid + NADH (yields) Alcohol + CO2 + NAD+
2. Lactic acid fermentation
In many cells, the pyruvic acid that accumulates as a result of
glycolysis can be converted to lactic acid.
a)
b)
Pyruvic acid + NADH (yields) lactic acid + NAD+
Lactic acid is produced in your muscles during rapid exercise
when the body cannot supply enough oxygen to the tissues.
c)
Without enough oxygen, the body is not able to produce all of
the ATP that is required.
d)
Your muscle cells rapidly begin to produce ATP by lactic acid
fermentation.
e)
Unicellular organisms also produce lactic acid as a waste
product during fermentation.
f)
Prokaryotes are used in the production of a wide variety of
foods and beverages, such as cheese, yogurt, buttermilk, and sour
cream.
g)
VI. The Krebs Cycle and Electron Transport
A. At the end of glycolysis, about 90% of the chemical energy that
was available in glucose is still unused, locked in the high-energy
electrons of pyruvic acid.
B. The Krebs Cycle
1. Oxygen is required for the final steps of cellular respiration.
2. This is an aerobic process.
3. The energy-releasing pathways within cells require oxygen, and
that is the reason we need to breathe, to respire.
4. In the presence of oxygen, pyruvic acid produced in glycolysis
passes to the second stage of cellular respiration, the Krebs cycle.
5. During the Krebs cycle, pyruvic acid is broken down into carbon
dioxide in a series of energy-extracting reactions.
6. Krebs begins when pyruvic acid produced by glycolysis enters the
mitochondrion.
7. One C atom from pyruvic acid becomes part of a molecule of CO2,
which is eventually released into the air.
8. The other C atoms are joined to a compound called coenzyme A to
form acetyl-CoA.
9. Acetyl-CoA then adds the C-C actyl group to a 4-C molecule,
producing a 6-C molecule called citric acid.
10. Citric acid is broken down into a 4-C molecule and electrons are
transferred to energy carriers.
11. What happens to the Krebs cycle products?
a)
CO2 is released when you respire.
ATP produced directly in the Krebs cycle can be used for
cellular activities.
b)
In the presence of oxygen high-energy electrons, NADH, can be
used to generate huge amounts of ATP.
c)
C. Electron Transport Chain
1. The Krebs cycle generates high-energy electrons that are passes
to NADH to FADH2.
2. The electron transport chain uses the high-energy electrons from
the Krebs cycle to convert ADP to ATP.
3. Every time a pair of high-energy electrons moves down the
electron transport chain, the energy is used to move H+ across the
membrane.
4. These ions then rush back across the membrane, producing
enough force to spin the ATP synthase and generate enormous
amounts of ATP.
VII.
The Totals
A. How much chemical energy does cellular respiration yield from a
single molecule of glucose?
1. Glycolysis produces 2 ATP
In the absence of oxygen that is all the energy that a cell
extracts from glucose.
a)
2. Krebs cycle and electron transport chain produce 34 ATP
3. The 36 ATP molecules the cell makes per glucose represent about
38% of the total energy of glucose.
4. What happens to the remaining 62%.
VIII. Energy and Exercise
A. Quick Energy
1. In terms of a race when the gun goes off the runner’s muscles
contain enough ATP for a few seconds of intense activity.
2. Before most runners have passed the 50-meter mark, that store of
ATP is nearly gone.
3. At this point, their muscle cells are producing most of their ATP by
lactic acid fermentation.
4. These sources can usually supply enough ATP to last about 90
seconds.
5. Fermentation produces lactic acid as a byproduct. When the race
is over the runner breaths heavily to pay back the oxygen dept.
B. Long-Term Energy
1. For exercise longer than about 90 seconds, cellular respiration is
the only way to generate a continuing supply of ATP.
2. Your body stores energy in muscle and other tissues in the form
of the carbohydrate.
3. This is usually enough to last for 15 or 20 minutes of activity.
4. After that, your body begins to break down other stored
molecules, including fats, for energy.